1. Field of the Invention
The present invention relates to novel thermosetting imido prepolymers, and, more especially, to low melt viscosity imido prepolymers comprising the copolymerizates of at least one N,N'-bismaleimide, at least one hindered diprimary diamine, at least one comonomeric heterocycle, and at least one comonomeric acrylate.
2. Description of the Prior Art
In published French Application FR-A-No. 2,608,613, assigned to the assignee hereof, polymers containing imide groups are described which comprise the product of the reaction, at a temperature ranging from 50.degree. C. to 300.degree. C., among at least:
(a) one N,N'-bisimide or a combination of several bisimides having the formula: ##STR1## Wherein each of the symbols Y, which may be identical or different, is H, CH.sub.3 or Cl; the symbol A is a divalent radical selected from among the following radicals: cyclohexylenes; phenylenes; 4-methyl-1,3-phenylene-; 2-methyl-1,3-phenylene; 5-methyl-1,3-phenylene; 2,5-diethyl-3-methyl-1,4-phenylene-; and the radicals of the formula: ##STR2## wherein T is a single valence bond or one of the atoms or ##STR3## and each of the symbols X, which may be identical or different, is a hydrogen atom or a methyl, ethyl, propyl or isopropyl radical; and
(b) one or more hindered biprimary diamine(s) selected from among:
(i) the species corresponding to the general formula: ##STR4## wherein each of the symbols R.sub.1, R.sub.2, R.sub.3 and R.sub.4, which may be identical or different, is a methyl, ethyl, propyl or isopropyl radical; and each of the symbols Z, which may be identical or different, is a hydrogen atom or a chlorine atom; and
(ii) the species corresponding to the general formula: ##STR5## wherein the amino radicals are in a meta- or para-position with respect to one another; and each of the symbols R.sub.5, which may be identical or different, is a methyl, ethyl, propyl or isopropyl radical.
The steric hindrance in the diamines (b) results in weak reactivity of the components of the polymerization mixture, which permits the gelling of the prepolymers and the complete hardening of the resins to be achieved after longer periods of time than are required in the case of polyaminobismaleimides produced from unhindered diamines. This lower reactivity of the components of the polymerization mixture may be of particular interest, on one hand, for the production of molded items by simple casting of the prepolymers, on the other hand, for the production of shaped articles by the filament winding technique and, finally, for the production of preimpregnated intermediate articles using the technique known as "hot melt", in which fibrous materials are impregnated with molten prepolymers.
Furthermore, the polyaminobismaleimides according to the '613 French Application are prepared without taking any particular precautions with regard to health, hygiene and safety, since the hindered diamines (b) that are used do not present the toxicity risks generally associated with unhindered diprimary aromatic diamines.
The aforementioned French application also provides that in preparing the polymers, optional reactant (or reactants) (c) and/or (d) may be added to the reaction mixture in addition to the reactants (a) and (b), with (c) being one or more monomer(s) other than a bisimide of formula (I) and having one or more carbon-carbon polymerizable double bond(s), and (d) being an imidazole compound. Reactant (c) is added when some of the rheological characteristics and/or mechanical properties of the polymers require modification, whereas reactant (d) is added when the reactivity of components (a), (b) and, when appropriate, (c) in the mixture, has to be increased.
Over the course of investigation of the polyaminobismaleimides for particular use in the production of shaped articles by the filament winding technique and the solvent-free preimpregnation technique, a serious need has arisen for thermosetting polyaminobismaleimide prepolymers, the preparation of which requires no particular precautions with regard to health, hygiene and safety and which have such viscosities that the values and changes over time of which are perfectly suitable for the types of conversion involved in filament winding and hot melt impregnation operations.